THz detection has many applications. These applications include concealed weapon detection, surveillance cameras, astronomy, non-destructive material testing, as well as ample biological and medical applications. The most common THz detectors currently available are single, or sparse element scanning systems, which typically use heterodyne detection with high speed Schottky diode mixers.
The development of imaging micro-spectrometers operating in the visible, SWIR and MWIR regions of the optical spectrum have been advanced in the last few years through academic and industrial developments. For example, there has been a recently developed short-wave infrared (SWIR) micro-spectrometer based on integration of a parallel plate MEMS optical filter directly with an IR-detector. Monolithic integration of the MEMS filter with the detector is a preferred approach as this reduces the cost of assembly and test of these devices compared with their hybrid equivalents. Hybrid micro-spectrometers have been developed using HgxCd1-xTe based detectors, as these detectors may operate across the short-wave (1.6-2.5 urn), mid-wave (3-5 um) and long wave (8-12 um) regions of the IR spectrum with extremely high detectivity. A primary technical challenge in achieving the integration of a MEMS Fabry-Pérot filter with the HgxCd1-xTe detector is to keep the processing temperatures low.
A MEMS tunable filter may be monolithically integrated with the silicon focal plane electronics device substrate by addition of MEMS processing steps using industry standard wafer processing. For example, following the fabrication of a silicon focal plane device, an overlay metal interconnect/antenna and passivation layers, a lower mirror of a Fabry-Pérot etalon may be deposited using a thermal evaporation process. This lower mirror layer includes silicon compatible layers with dielectric constants consistent with good mirror reflection, such as germanium/silicon layers. This lower mirror layer may be patterned using standard photolithography processes.
A spacer layer is then deposited and patterned to provide mirror structural supports. A silicon nitride stack is then deposited using plasma enhanced chemical vapor deposition to form an upper mirror support and a flexible membrane for the etalon tuning structure. An upper mirror is then deposited on the silicon nitride stack in the same manner as fabrication of the lower mirror. The upper mirror is then patterned to a desired shape and the silicon nitride membrane is subsequently formed using a dry etching process. Metal electrodes are then deposited on top of the mirror to allow operation of the etalon via electrostatic force. Finally, the upper silicon nitride and mirror structure is released by etching away the spacer layer to form the air-gap for the Fabry-Pérot cavity.
In the THz regime (0.1-1.0 mm), however, the current state of art for spectral imaging systems are single point detectors (e.g. Shottky diodes) that are spectrally scanned through tuning of active laser sources, or through time delay spectroscopy using ultrafast laser sources. A 2-dimensional image is achieved by mechanically scanning the single point detector spatially to form an X, Y image. There is no prior art on a large format imaging focal plane array (FPA) with THz imaging detectors that also includes digitally tunable optical filters that are integrated onto a monolithic chip.
There are also many shortcomings with current THZ detectors. There is an ever present need for THz detectors with a higher quantum efficiency, a higher level of detector integration in low cost, non-bulky systems, and an improved signal-to-noise ratios (SNR).